The invention relates to the packing of polycrystalline silicon.
Polycrystalline silicon (polysilicon) is predominantly deposited by means of the Siemens process from halosilanes such as trichlorosilane and then comminuted with minimum contamination to polycrystalline silicon fragments.
For uses in the semiconductor and solar industries, chunk polysilicon with minimum contamination is desirable. Therefore, the material should also be packed with low contamination before it is transported to the customer.
Tubular bagging machines, which are suitable in principle for packing of chunk silicon, are commercially available. A corresponding packing machine is described, for example, in DE 36 40 520 A1.
Chunk polysilicon is a sharp-edged, non-free-flowing bulk material. It is therefore necessary to ensure in the course of packing that the material does not pierce or in the worst case even completely destroy the customary plastic bags in the course of filling and in the course of packing, in the course of transport and in the course of unpacking, especially since this would be associated with contamination of the polycrystalline silicon.
In order to avoid problems in the course of filling into plastic bags, the commercial packing machines have to be suitably modified for the purpose of packing polysilicon.
DE 10 2007 027 110 A1 discloses a method for packaging polycrystalline silicon, in which polycrystalline silicon is filled by means of a filling device into a freely suspended, completely formed bag, and the filled bag is subsequently closed, wherein the bag consists of high-purity plastic with a wall thickness of from 10 to 1000 μm, the filling device comprising a freely suspended energy absorber of a nonmetallic low-contamination material, which is introduced into the plastic bag before filling with the polycrystalline silicon and by way of which the polycrystalline silicon is filled into the plastic bag, and the freely suspended energy absorber subsequently being removed from the plastic bag filled with polycrystalline silicon, and the plastic bag being closed.
Such a process, which envisages an energy absorber within the plastic bag, can substantially prevent piercing of the plastic bag during the packing operation. However, this is only true of small or lightweight fragments.
It has been found that the risk of bag damage increases proportionally to the fragment mass.
One way of reducing the piercing rate which is conceivable in principle, by using a thicker bag film, has not been found to be very practicable, especially since such a less flexible film would be difficult to handle.
Such piercing of the bag, however, can occur not only during packing, but also in the course of transport to the customer. Chunk polysilicon is sharp-edged, and so, in the event of unfavorable orientation of the fragments in the bag, relative movement of the fragments with respect to the bag film, or pressure of the fragments thereon, can cause them to cut through or pierce the bag film.
Fragments protruding from the bag packing can directly be contaminated unacceptably by surrounding materials, and fragments within by incoming ambient air.
In this regard, no solutions are known in the prior art.
These problems gave rise to the objective of the invention.